This invention relates to a method for the manufacture of a low thermal expansion cordierite ceramic.
Generally, cordierite is a mineral having a composition represented by 2MgO.2Al.sub.2 0.sub.3.5SiO.sub.2. It exhibits a very low thermal expansion coefficient over a wide range of temperatures and offers outstanding resistance to thermal shocks exerted by abrupt temperature changes. Owing to its excellent thermal stability, therefore, cordierite ceramics are widely adopted as refractory heat plates, as thermally resistant materials in laboratories, and as raw material for honeycomb structure bodies in heat exchangers of the recently developed type.
The sintering temperature at which cordierite is converted effectively into a ceramic is limited to a very narrow range. The practice of widening the range of sintering temperature by adding alumina, zirconia compound, or aluminum titanate to cordierite has found widespread acceptence for the purpose of eliminating the disadvantage due to the limited firing temperature range (Yoichi Shiraki: "Industrial Ceramics," page 313, Gihodo). Various studies are being made on the lowering of the thermal expansion coefficient of the sintered product of cordierite, namely cordierite ceramics.
One example of a technique for improving ceramics by the use of zircon is described here.
Since zircon avoids appreciably increasing the thermal expansion of cordierite ceramics and, moreover, enjoys thermal stability, the practice of adding this substance to cordierite has been gaining acceptance and yielding favorable results (Research Notes of the Kyoto Municipal Research Institute of Industry, page 3, June, 1978.) The addition of zircon, however, entails a disadvantage that the thermal expansion of the cordierite ceramics increases in proportion as the proportion of zircon to be added to the total amount of cordierite is increased. Even when the zircon to be added is as finely divided as the commercially available zircon flour, it is difficult for the cordierite to be sintered to an extent such that the produced product, namely cordierite ceramics, will assume a state having absolutely no water absorption coefficient. In the case of zircon sand now in popular use, since it contains iron as an impurity and has high hardness, cordierite incorporating such zircon sand cannot produce good cordierite ceramics.
Method for producing low thermal expansion cordierite have been produced by U. S. Pat. Nos. 4,033,779 and 4,194,917.
The former method resides in incorporation of Al.sub.2 O.sub.3.
The latter method resides in incorporation of Y.sub.2 O.sub.3, CeO.sub.2, and La.sub.2 O.sub.3.
With these methods however, the following faults are found. In the former method, regulation of particle size is difficult. In the latter method, the rare elements are expensive.
An object of this invention is to provide an inexpensive method for the manufacture of a low thermal expansion cordierite ceramics which exhibits a very low thermal expansion coefficient and yet retains the outstanding thermal shock resistance of the cordierite.